Water rods having a non-circular cross section have been proposed for BWR fuel bundles. Many cross sectional shapes could be used. Three shapes are described with emphasis on a "peanut" shape.
The use of a non-circular cross section creates problems in locating spacers on the water rod, and also creates opportunities for simplifying the water rod construction.
The "peanut" shape is used as an illustrative example.
The origin of this "peanut" cross section can be best understood when the displacement of fuel rod positions by large water rods is understood with respect to a matrix of upstanding fuel rods within a fuel bundle for a boiling water nuclear reactor.
Nuclear fuel bundles utilized in boiling water nuclear reactors can be simply summarized. Specifically, such bundles include a matrix of upstanding fuel rods. These fuel rods are supported on a lower tie plate, extend to and toward an upper tie plate, and generally define a matrix of rows and columns of vertical fuel rods. The lower tie plate--in addition to the supporting of the fuel rods--permits the entry of water. The upper tie plate--in addition to maintaining the matrix of fuel rods upright from the lower tie plate--permits the discharge of heated water and generated steam from the interior of the fuel bundle. A channel surrounds the fuel bundle from the lower tie plate to the upper tie plate. This channel confines fluid flow from the lower tie plate to the upper tie plate while maintaining the flow path in the interior of the fuel bundle separate and apart from the so-called core by pass region exterior of the fuel bundle.
It is important to the fuel bundle that the matrix of upstanding, long slender fuel rods be reinforced interior of the fuel bundle. This reinforcement enables the fuel rods to maintain their designed side-by-side relation for efficient nuclear reaction as well as avoid vibrating contact one with another. Specifically, the fuel rods must be held in place at intervals along the length of the fuel bundle. This is accomplished by so-called spacers placed at intervals along the length of the fuel bundles. These spacers form a cell matrix around each individual fuel rod and thus maintain the fuel rods in their designed side-by-side relationship against the forces of flow induced vibration and rod bow.
Operation of the fuel bundle is simple to understand. Water enters the fuel bundle through the lower tie plate. The water acts both as a coolant and a moderator. Acting as a coolant, the water is heated and turned to steam in the fuel bundle by heat generated in the fuel bundle. Acting as a moderator, fast neutrons from the nuclear reaction are moderated and become slow moving or thermal neutrons to continue the chain reaction in the interior of the fuel bundle. As a result, steam generating reaction continues in a core of many fuel bundles in the boiling water nuclear reactor.
When operating, such fuel bundles have increasing fractions of steam in the upper two phase portion of the fuel bundle. These increasing fractions of steam take away needed moderator in the upper two phase region of the fuel bundle. At the same time, moderator is needed in the upper two phase region of the fuel bundle. To remedy this lack of moderator in the upper two phase region of the fuel bundle, large water rods have been used. The function of these large water rods can be summarized.
Simply stated, the term "large water rod" is a term of comparison. By comparison to the fuel rods, the large water rods have an increased diameter. The water rods receive water from the lower single phase (water) region of the fuel bundle (either through the tie plate or from the water immediately above the tie plate) and transport that water without boiling to the upper two phase region of the fuel bundle. When the water resident in the large water rod reaches the upper two phase region of the fuel bundle, it increases the so-called "fuel to moderator" ratio in the upper two phase region of the fuel bundle. As a result, greater moderation of the fast neutrons to their thermal state occurs, and a more efficient fuel bundle is obtained.
In order to maximize the moderating effect of the large water rod, and to minimize bypass flow area, the water rod should fill as much of the available space as possible. When two or more fuel rods are removed from a square array, the resulting area can only be partially filled by a circle. For example, in prior art 8.times.8 fuel lattice, 4 central fuel rods were removed and were replaced by a circular water rod. In this case the circle fills most of the square area resulting from the removal of four fuel rods.
In a prior art 9.times.9 fuel bundle, it was found that an optimum nuclear design resulted when seven fuel rods were removed. In this case two large water rods were used, and the fraction of the area occupied by the two water rods was smaller. A substantial gain in neutron moderation and reduction in bypass flow area can be obtained by using a single water rod of non-circular cross section. However, a noncircular water rod cross section creates problems in the assembly of a fuel bundle and the location and locking of the spacers to the fuel bundle.
One of the functions of a water rod is to hold the spacers in their correct axial position. Unfortunately, the spacer locating method used with circular water rods cannot be used with a non-circular cross section. The invention described herein provides alternate methods for locating the spacers with a non-circular water rod cross section.
Fuel bundles having large water rods are typically given an assembly sequence which features the use of the water rods in aligning the fuel bundle spacers to receive the individual fuel rods within the fuel rod matrix. Specifically, the spacers are typically held in their assembled spaced apart alignment in a jig. Once the spacers are in place, the large water rod is assembled to the spacers and locked with respect to the spacers. Such locking typically occurs by rotating the water rod relative to the spacers in a jig until a system of tabs or springs locks the water rod and spacers into a "tree" type configuration. In this tree type configuration, all of the cells for holding the individual fuel rods are aligned. Typically, the lower tie plate is inserted at one end of the water rod and spacer assembly which is to become the fuel bundle.
Thereafter, the fuel rods are inserted to the jig held spacers. Each fuel rod is successively threaded through the aligned cells of the spacers to a position of penetrating support on the lower tie plate. Such insertion continues until all fuel rod positions are occupied. Thereafter--and as one of the final steps of assembly - the fuel rod channel is placed about the fuel bundle.
It is important to note, that in the prior art in order to effect the locking of the spacers to the large water rod, rotation of the large water rod had to occur relative to the spacers.
Variations have been used in the cross sectional configuration of water rods. All these variations take advantage of some portions of the water rod section being round. These variations have included the use of two round water rods. As most pertinent to this disclosure, such variations include having so-called double "D" shaped water rods.
In the so-called double "D" water rod, a configuration not unlike a peanut sectioned water rod is eventually generated. Typically, two water rods each having a "D" cross section are placed back to back.
"D" sectioned water rods have a basic circular configuration. These "D" sectioned water rods a truncated by a chord in their cross section. Typically the chord cuts off something less than one half of the total diameter of the water rod. A fat "D" section to each one half of the pair of water rods results.
Placement of two such "D" sectioned water rod occurs within the fuel bundle. The linear portion of the "D" section on one water rod is placed back to back with the linear portion of the "D" section on the other water rod. Since the linear portion of the "D" section occurs along a chord leaving more than one half the circular section of the water rod intact, the two water rods when placed back to back have a total cross section not unlike a "peanut" sectioned water rod.
Remembering that the "D" sectioned water rod is basically circular, it is still possible to use rotation of one "D" sectioned water rod is utilized to lock the fuel rod spacers in place. Specifically, excepting for the linear portion of the "D", such fuel rods are generally round. Typically a first "D" sectioned water rod is placed. Thereafter, this first "D" sectioned water rod is rotated to lock the spacers through which the water rod is threaded into place. This locking of the spacers disposes the "D" sectioned water rod in its final rotational alignment. The remaining "D" sectioned water rod is then moved into place. This second water rod has its linear section at the "D" cross section confronted to the linear section at the "D" of the first placed water rod. As a result, both water rods are locked in rotational alignment with the spacers keyed to the first placed "D" sectioned water rod.
The double "D" water rod contains the double metallic boundary between the two linear sections of the "D". This being the case, unnecessary absorption of neutrons occurs. Because of this unnecessary neutron absorption, the so-called "peanut" sectioned water rod is preferred because of its lesser neutron absorption.
Unfortunately, the "peanut" sectioned water rod does not lend itself to easy rotation. Locking of the spacers to the section of the water rod is complicated. It is believed that this is a principal reason that such water rods have not been utilized heretofore in fuel bundle designs.
It is further to be noted that large water rods have to be connected to the fuel bundles--typically at the upper and lower tie plates. Such attachment has in the past included a small sectioned conduit from the lower tie plate to the larger sectioned water rod. This small sectioned conduit it provided to enable necessary movement of the large water rod with respect to the fuel bundle. For example, seismic design requires that such a flexible connection be utilized.
Typically, the large cross section water rod begins at a location considerably above the tie plate. Further, it has been the practice in the past to spring load the large water rod downward toward and onto the lower tie plate to secure the large water rod in the fuel bundle assembly. While such spring loading serves to secure the large water rod, it has the disadvantage of requiring an assembly procedure in which many parts are utilized. Further, these additional parts contribute to the neutron absorption of the fuel rod.